1. Field of the Invention
The present invention concerns communication systems in which data can be transferred between a plurality of stations.
2. Description of Related Art
Existing central control architecture for such systems of which an example is System X offers essentially two communication interfaces. These are intercluster serial communications and the parallel direct 1/0. These interfaces correspond to the traditional separation of control elements from applications hardware. In this traditional view there are small numbers of complex, expensive control elements controlling large numbers of simple, cheap peripherals. A symmetric asynchronous message interface is provided between the data processing control elements, and these elements in turn communicate with application hardware via a synchronous low-level word by word mechanism. Thus the application hardware elements can only communicate with each other via the control elements.
With the development of cheap processing power this architecture is not suited to modern requirements and has become a constraint on future evolution. In particular the availability of microprocessors and cheap memory has made it much simpler to enhance the capability of peripheral devices so that more and more functions are being located at the peripherals. Thus a typical interaction between a control element and a peripheral now requires tens of bytes of parameter information. Moreover since the peripheral does much more the length of the interaction has increased. In order to carry this increased traffic using current interfaces there has now been implemented asynchronous message passing arrangements on top of the message passing mechanisms which were already provided. This is inefficient in that it holds up both ends of the communication more than necessary and requires a wholly unnecessary speed of response from the peripherals. Furthermore there is now, and in the future this will increase substantially, a growth in traffic transmitted between peripherals. In this context the central control becomes a bottleneck.
In view of the above it is probable that future systems will involve a large number of duplicated stations which are capable of exchanging messages of fairly similar lengths asynchronously, on a peer-to-peer basis and at high rates.
In such a system the number of terminals is likely to be over 100 with the possibility of handling still more. There are two alternative basic architectures which can be used. One is a star-network and the other involves a ring topology. Star-type topologies have great difficulties with high numbers of stations at the required bandwidth, and are vulnerable to failure of the central control element. On the other hand the ring topology presents the problem that one failure in a simple ring can be fatal. There have been a number of proposals to overcome this problem since the functioning of the whole system depends on the reliability of the ring.
One method of increasing reliability is to have two concentric ring structures with each terminal being capable of communicating with both rings so that should one ring fail the other can take over its function. Normally the rings operate in a contra-rotating mode with the data in one ring being transmitted in a direction opposite to the direction of transmission in the other ring. The ring topology is particularly suited to the transmission of data by optical fibre although the present invention is not limited solely to optical rings. In such a topology one channel or ring is known as the primary channel and the other as the secondary or reserve.